Seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the and surface or seafloor. Among other things, seismic data acquisition involves the generation of acoustic waves and the collection of reflected/refracted versions of those acoustic waves to generate the image. This image does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process in the field of seismic surveying.
A significant problem in marine-based seismic data analysis is receiver ghosts. In marine-based seismic data acquisition, the up-going acoustic waves reflected from subsurface reflectors are first recorded by the receivers. Next, the acoustic waves continue to propagate to the surface where they are reflected back down and are recorded again by the receivers as ghosts. The reflectivity at the free surface is close to negative one and based on this property, the down-going acoustic waves have similar amplitudes as the previously described up-going acoustic waves but have an opposite polarity. Accordingly, some of the frequencies in the recorded acoustic wave data are attenuated near the ghost notches and the removal of the receiver ghosts can provide the benefit of infilling the ghost notches and providing higher quality images in terms of frequency band and signal-to-noise ratio.
Removing receiver ghost before data migration has proven advantageous because it provides better low frequency and high frequency response as well as a higher signal-to-noise ratio for preprocessing steps, e.g., multiple suppression and velocity analysis. In one attempt to remove receiver ghosts, associated with receivers maintained at a constant depth, the ghost removal has been carried out in the frequency/wavenumber (FK) domain but limitations such as requiring a constant depth for the receivers and limiting the seismic data collection to two dimensions because of the coarse sampling in the crossline direction for the high frequencies in the seismic data have provided unsuitable results. For an example, please refer to J. T. Fokkema and P. M. van den Berg in their 1993 article entitled “Seismic Applications of Acoustic Reciprocity” published by Elsevier and incorporated herein by reference.
In another attempt to remove receiver ghosts associated with non-horizontal receiver based seismic data, a method was presented by C. D. Riyanti, R. G. Van Borselen, P, M. van den Berg and J. T. Fokkema in their 2008 article entitled “Pressure Wavefield Deghosting for Non-horizontal Streamers,” published in the 78th Meeting, SEG, Expanded Abstracts, pages 2652-2656 and incorporated herein by reference. The presented method was capable of handling variable-depth receivers as long as their depths were accurately known, but as above, could handle only two-dimensional data because the method worked in the FK domain.
A further attempt to remove receiver ghosts includes a method using both pressure wavefields and velocity wavefields to attenuate the receiver ghosts. In this method the particle velocity is measured in the vertical direction of the wave propagation. In essence, the upward moving waves detected by the geophones and hydrophones are in phase and the downward moving reflections, i.e., the receiver ghosts, are one hundred eighty degrees out of phase so that summing the two recorded datasets can attenuate the receiver ghost. Unfortunately, difficulties arise in calibrating the difference between the two datasets because of low signal-to-noise ratio for particle velocity data and emergence-angle variations. This method is described in more detail in a 2007 article by D. A. Carlson, W. Long, H. Tobti, R. Tenghamn and N. Lunde entitled “Increased resolution and Penetration from a Towed Dual-Sensor Streamer,” published in First Break, 25, pages 71-77 and incorporated herein by reference.
Other attempts have been made to remove receiver ghosts, for example, the interested reader is referred to B. J. Postumus who authored a 1993 article entitled “Deghosting Using a Twin Streamer Configuration,” published in Geophysical Prospecting, 41, pages 267-286 for concurrently towed shallow and deep streamers, and enhancements to this method by A. K. Özdemir, P. Caprioli, A. Özbek, E. Kragh and J. Robertsson for their 2008 article entitled “Optimized Deghosting of Over/Under Towed-Streamer Data in the Presence of Noise,” published in The Leading Edge, 27, page 90 for an optimal deghosting approach in the FK domain to jointly deghost the shallow depth data and the deep depth data and by B. Gratacos for the 2008 article entitled “Over/Under Deghosting: 1D, 2D or 3D Algorithms in the F, FK or FXY Domains,” published in the 78th Meeting, SEG, Expanded Abstracts, pages 125-129 to obtain an upward direction wavefield. Unfortunately, this method and its enhancements suffer collectively from sparse cross-line sampling and require accurate receiver positioning, not easily accomplished, for high frequencies.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, and which generate improved fold (illumination) maps.